Metallization of carbon nanotubes (CNTs) presents a next-generation nanotechnology for many important applications, such as fuel cells, electrochemical sensors, CNT alignment and patterning, assessment of CNTs' structural defects, electromagnetic interference shielding, and the like. Metallized CNTs (mCNTs) also offer unique solutions to problems encountered in nano-reinforced composites. Further explorations in these areas demand advances in the development of mass-production techniques for the production of mCNTs.
Although the metallization of nanotubes has been accomplished by previous methods, each method has limitations affecting its commercial feasibility. For example, physisorption and electroless plating have both been previously used to deposit metal nanoparticles on CNTs, but both methods utilize an oxidative acid pretreatment step to create additional sidewall defects in the CNTs prior to metal nanoparticle attachment. The additional CNT sidewall defects act as either attachment sites (physisorption) or nucleation sites (electroless plating) to achieve metallization. However, sidewall defects are known to degrade the mechanical and electrical properties of CNTs and the oxidative acid treatment to create the defects is, thus far, a time-consuming and uncontrolled process. In addition, the physisorption technique requires a separate preparation of metal nanoparticles prior to a lengthy sonication process in order to disperse and attach the metal particles onto CNTs.
The electroless plating method often requires a complicated activation-sensitization procedure to prepare the CNT surface for metal depositions. The harsh acid treatment, the extended sonication, the activation-sensitization procedure, and certain galvanic displacement reactions are very disruptive to the intrinsic structure and properties of CNTs. In addition, physisorption and electroless plating processes often result in chunky metal particles (≧50 nm in diameter) mounted on the CNT surface, where severe dislodging is often observed due to the large size of the metal particles and the relatively loose attachment.
It remains exceptionally challenging to achieve reliable electrical contact with bulk CNT samples, an important step for accomplishing electrochemical deposition. Previous efforts to solve this problem include growing CNTs on conducting substrates, microlithography, electrophoresis, sputtering, or thermal evaporation. These processes have been largely unsuccessful, especially at producing reliable, large-scale (grams) amounts of metallized CNTs. Therefore, there remains a need for a scalable process which also provides good control of depositing varying morphologies of metal nanostructures on CNTs, from discrete atom clusters to continuous coatings.